Cyclic AMP produces shape and motility changes in cultured cells, presumably through effects on contractile proteins such as actin and myosin. We have studied the arrangement of these contractile proteins and found that cyclic AMP produces ordered bundle arrays of actin-containing microfilaments and of microtubules. The intracellular arrangement of myosin under these conditions has been studied by immuno-cytochemical and biochemical methods. Myosin is associated with microfilament bundles, the soluble cytoplasm, other cell organelles, and the plasma membrane. We have found that myosin is exposed at both the inside and the outside of the plasma membrane and associated with another new membrane protein. The surface microstructure of these cells has also been studied with cyclic AMP treatment and the changes in agglutination by plant lectins have been linked to changes in surface microvilli caused by cyclic AMP. Since malignant cells in culture often have low cyclic AMP levels, disorganized contractile protein elements, and rapid cell movement, the control of contractile protein functions by cyclic AMP is likely to be highly significant in understanding the nature of malignancy.